1. Field of the Invention
This invention relates generally to the field of gas laser technology, and more particularly to a ceramic waveguide gas laser operated at relatively high pressures.
2. Description of the Prior Art
Thermally stabilized laser apparatus is known to those skilled in the art, typical examples of which are disclosed in U.S. Pat. Nos. 3,699,472, Young; 3,588,737, Chow; and 3,528,206, Baird. Gas laser tubes constructed of mullite, zircon, alumina and beryllia are also disclosed in U.S. Pat. No. 3,492,597, Neusel. Additionally, a glass ceramic charcterized by a zero coefficient of thermal expansion over a broad temperature range has been disclosed in a publication entitled "Laser Components of Cer-Vit" which appeared in the SPIE INFORMER on June, 1971, at page G39.
Discharges in small bore tubes using gases with homogeneously broadened laser transitions obtainable with mixtures of CO.sub.2, CO and N.sub.2 O permit operation at higher pressures. Pressure broadened optical gain lines are desirable inasmuch as they allow wider frequency tuning of a laser oscillator and they allow small amplitude variations when operating over a relatively wide range of frequencies near gain line center. It is also known that optimum pressures for laser action in wall dominated discharges are inversely proportional to the bore diameter. This makes it desirable to build laser tubes with small bores or channels; however, in constructing such laser systems for optimum operation, certain problems are encountered. First of all, diffraction of the optical beam from the output end of the bore or channel commonly referred to as the "guide" end can lead to large diffractional losses unless curved mirrors are precisely positioned from the guide end to minimize such losses. Alternatively, flat mirrors which terminate the guide end reduce such diffraction losses to nearly zero while greatly improving mechanical stability by making the mirrors integral parts of the optical gain chamber. Secondly, small bores or channels introduce optical absorption losses and losses due to scattering from the wall surfaces. Thirdly, operation of small bore tubes reduces wall surface area. Noting that the optimum pressure is inversely proportional to the bore diameter while the area of the tube varies as the square of the wall diameter, the reduction of the heat transfer area makes heat removal a more serious problem assuming that the electrical input power per unit length remains constant as the tubes are scaled to smaller diameters.
Optical radar and communications systems are known to use gaseous laser transmitters operating near 10 .mu.m. For such purposes as well as for certain laboratory uses, it is desirable that compact frequency and amplitude stable gas lasers having good beam qualities, high efficiency as well as frequency tunability. Accordingly, mixtures of carbon dioxide (CO.sub.2), carbon monoxide (CO) and nitrous oxide (N.sub.2 O) are utilized. Such lasers, however, operate most efficiently at the lowest possible gas temperatures and include discharge chambers constructed of high thermal conductivity, electrically insulating ceramics which allow for efficient heat transfer. Beryllium oxide (BeO) and alumina (Al.sub.2 O.sub.3) have heretofore had wide usage since they include these properties. These materials, however, have relatively large thermal expansivities which give rise to thermal frequency variations. Such laser frequency variations are important. They come about when temperature changes cause thermal expansion or contraction of the mirror separation of the laser cavity, causing the Fabry Perot resonance frequency of the mirror cavity to change. It can be shown that the frequency excursion .DELTA. f of a small laser oscillator due to a temperature change .DELTA. T is given by the equation: EQU .DELTA.f = (c/.lambda.).alpha. .DELTA.T (1)
where c is the velocity of light, .lambda. is the optical wavelength and .alpha. is the thermal expansion coefficient of the laser cavity material. Thus for a given temperature change, the laser frequency change is proportional to the expansivity of the cavity. Accordingly, if one constructs a waveguide laser with flat end mirrors terminating the guide ends, i.e. the bore or channel end which acts to minimize diffraction coupling losses and mechanical frequency fluctuations, then thermal frequency changes become highly significant. For typical operating pressures gains and tube lengths, thermal cavity length changes can cause sufficiently large changes in the Fabry Perot resonance that the laser "jumps" from one optical gain transition line to another. This leads to gross laser frequency and amplitude changes.